The present invention relates to a coated cutting tool insert particularly useful for turning of steel, like low alloyed steels, carbon steels and tough hardened steels, at high cutting speeds.
High performance cutting tools must nowadays possess high wear resistance, high toughness properties and good resistance to plastic deformation. This is particularly so when the cutting operation is carried out at very high cutting speeds and/or at high feed rates when large amount of heat is generated.
Improved resistance to plastic deformation of a cutting insert can be obtained by decreasing the WC grain size and/or by lowering the overall binder phase content, but such changes will simultaneously result in significant loss in the toughness of the insert.
Methods to improve the toughness behaviour by introducing a thick essentially cubic carbide free and binder phase enriched surface zone with a thickness of about 20-40 .mu.m on the inserts by so called gradient sintering techniques are in the art.
However, these methods produce a rather hard cutting edge due to a depletion of binder phase and enrichment of cubic phases along the cutting edge. A hard cutting edge is more prone to chipping. Nevertheless, such carbide inserts with essentially cubic carbide free and binder phase enriched surface zones are extensively used today for machining steel and stainless steel.
There are ways to overcome the problem with edge brittleness by controlling the carbide composition along the cutting edge by employing special sintering techniques or by using certain alloying elements, of which U.S. Pat. Nos. 5,484,468, 5,549,980, 5,729,823 and 5,643,658 are illustrated.
All these techniques give a binder phase enrichment in the outermost region of the edge. However, inserts produced according to these techniques often obtain micro plastic deformation at the outermost part of the cutting edge. In particular, this often occurs when the machining is carried out at high cutting speeds. A micro plastic deformation of the cutting edge will cause a rapid flank wear and hence a shortened lifetime of the cutting inserts. A further drawback of the above-mentioned techniques is that they are complex and difficult to fully control.
U.S. Pat. Nos. 5,786,069 and 5,863,640 disclose coated cutting tool inserts with a binder phase enriched surface zone and a highly W-alloyed binder phase.